Why look for a tool you have to bolt to a wall and you can't fit into your daughter's bag. I have a better idea for less cost.
Read this then improve on the design before you start put it all together.
1. Take a brand new softball and drill a 5/16 "polar" hole through it.
2. Put a 5/16 brass grommet in both holes
3. Thread a 5/16 nylon rope through it and tie the ends together forming an approximate 18-20 diameter loop or so the caribener is hanging 8" or so from the softball when the loop of the rope is pulled around the wrist and the ball is in the hand.
4. Attach a small carabiner to the loop.
5. Take three tennis balls and use three eyelet bolts the same diameter of the tennis ball
6. Cut small "polar" holes in each tennis ball
7. Fill each tennis ball with three different weights, I used nickles, another coach used metal bolts/nuts.
8. Push the eyelet bolt through the ball and attach using a washer and a nut.
9. Attach each tennis ball to the carabener, tighten the rope and have your daughter swing the softball around in the same pitching motion and you will see instant feedback of her arm angle.